Electrical Time-Domain Reflectometry (ETDR) uses the propagation of electromagnetic wave to determine the location and nature of various reflectors. Information is derived from reflections of a voltage pulse sent through a transmission medium, namely a transmission cable.
Electrical time domain transmission (ETDT) is another technique that has been used to monitor conductors. In electrical time domain transmission the coupling of two waveguides is monitored. Electrical time domain transmission is used in the semiconductor industry to measure and test circuit responses.
In ETDR, a voltage pulse generated by a signal generator is sent through a transmission cable, and reflections of the voltage pulse are sensed with a time domain reflectometer (TDR) sampling head. When the pulse encounters any discontinuity, for example, partial loss of a cross section of the cable, a portion of it is reflected back towards the sampling head. The arrival time and amplitude of the reflected signal include the information on distance between the points of monitoring and discontinuity, and the severity of damage at the discontinuity point, respectively. The directly measurable parameter, reflection coefficient, represents the percentage of the reflected wave to an incident wave or the change in characteristic impedance of the transmission medium.
ETDR has been developed and used for monitoring the health of cables themselves, such as the cables used in communication systems. Since the 1950s, power and telecommunication industries have used ETDR to locate and identify faults in transmission cables. ETDR has also been applied to a limited extent in other industries and there is interest in using ETDR and sensor cables to monitor other structures. For example, geotechnics has used ETDR to monitor rock masses or other geotechnical facilities that often undergo significant movement in a widespread area. There have also been attempts to monitor other structures, but it is believed that known techniques have produced a small signal-to-deformation ratio. The lack of sensitivity to deformation of commonly used commercial cables greatly limits the general applicability of ETDR for sensing defects in structures, such as structural members in buildings, bridges, roads and the like.